One of the major advances in communications in recent years has been the increased use of optical fiber systems for carrying large quantities of information with low distortion and low cost over great distances.
Optical delay lines are important components in optical communications for switching streams of optical signals from one path to another. For example, optical delay lines are critical components in packet switching used in Internet applications. Delay lines are also the building blocks in the correlation filters used to recognize packet addresses. They are also used in the buffers which control the timing of signal routing. Delay lines are used for synchronizing data bits, for compensating differential dispersion between different optical paths, and for compensating differential delay between paths. Because of the dynamics of high-speed systems, different delays may be required at different times.
A variable optical delay line typically comprises a plurality of optical fibers, each cut to a slightly different length, and a fiber switch to switch an input signal among them and thus control the delay. In principle this should work, but in practice it has serious shortcomings for high-speed systems. Cutting fibers to a precision of millimeters is difficult, and this difficulty limits the achievable accuracy of delay time. Moreover, a variable delay line having a large dynamic range will require many precisely cut fibers. It is not practical to provide sufficient precisely cut fibers to provide fine delay increments (high temporal resolution) over a large dynamic range. Accordingly there is a need for an improved variable optical delay line capable of providing high resolution over a large dynamic range.